1. Field of the Invention
The present invention relates to catalytic processes for conversion of syngas to alcohols. More particularly, it relates to molybdenum-based catalysts, supported on nanodimensional carbon materials, showing improved selectivity toward higher alcohols.
2. Background of the Art
A variety of alcohols, particularly those ranging from methanol to hexanol (C1-6OH), are products that can be synthesized from synthesis gas. This gas, also called “syngas,” is a mixture of hydrogen gas and carbon monoxide gas (H2/CO)). The C1-C6 alcohols in particular are considered to be important synthetic fuels and chemicals. In general, alcohols which are defined herein as “higher alcohols,” i.e., alcohols having at least two carbon atoms (C2+), are currently sought to serve as, in particular, automobile fuels and fuel blends. In this application many offer desirably high octane numbers as well as desirably low emissions of nitrogen oxide (NOx), ozone, CO, and aromatic vapors. In addition, the higher alcohols (C2+OH) may be useful as alternative feedstocks for commercially significant olefins (produced via dehydration of the higher alcohol), particularly when the syngas is derived from biomass or coal.
Unfortunately, not all conversion processes provide desirable selectivity to specific higher alcohols. Researchers have identified various means and methods to alter the selectivity. One obvious way is to use different catalysts. Known catalyst compositions to produce alcohols have included combinations of copper, zinc oxide, and alumina (Cu/ZnO/Al2O3); molybdenum sulfide (MoS2); cobalt and copper (Co—Cu); rhenium (Rh); and molybdenum carbide (Mo2C). Each of these will produce various combinations of selectivities. Among these useful catalysts is, for example, the catalyst disclosed in U.S. Pat. No. 5,627,295, which is based on copper-aluminum-zinc (Cu—Al—Zn) and another element selected from Group IIIA, IIIB, IVA, or IVB. U.S. Pat. No. 7,449,425 discloses using catalysts which comprise a noble metal and certain transition metals on magnesium-aluminum (Mg—Al) hydrotalcite supports.
Molybdenum-based catalysts (e.g., MoS2, Mo2C), including MoS2 promoted by alkali metal ions, have also been shown to improve ethanol selectivity. For example, U.S. Pat. Nos. 4,661,525 and 4,825,013 disclose methods for making a mixture of C2-C6 aliphatic alcohols by reacting carbon monoxide with hydrogen over alkali-doped MoS2 catalysts modified by the addition of an element of Group VIIIB (e.g., Co, Fe, or Ni). Alkali metal promoters, such as sodium (Na), potassium (K), cesium (Cs), or rubidium (Rb), mixed with cobalt-molybdenum-sulfur (Co—Mo—S) compositions have also been investigated for syngas conversion. U.S Patent Application 2010/007583718 describes a way to intercalate a strong basic promoter such as K, Cs, barium (Ba), strontium (Sr), scandium (Sc), lanthanum (La), or cerium (Ce) into layered MoS2 to prepare a basic catalyst for conversion of syngas. U.S. Pat. No. 4,675,344 discloses manipulation of hydrogen sulfide (H2S) concentration in the syngas feed to improve selectivity to higher alcohols versus methanol. U.S. Pat. No. 4,749,724 discloses use of alkali-doped MoS2 supported on activated carbon having a Brunauer-Emmett-Teller (BET) surface area ranging from 100 to 1,500 square meters per gram (m2/g).
Recently, researchers working in the field of materials chemistry have been successful in synthesizing new nanostructured materials, such as Mg—Si fishbone-like oxides, MgO nanosheets, nanoporous aluminophosphates, nanotubular titania and silica, nickel phyllosilicate nanotubes, and carbon nanosheets, nanofibers, and nanowhiskers. Some initial attempts to employ carbon nanotubes, mesoporous carbon structures, and carbon nanofibers as suitable supports for the conversion of syngas to liquid fuels have been recently disclosed. In the art it is common for the terms “nanoporous” and “mesoporous” to be used interchangeably to refer to the presence of nanodimensional pores.
Despite such attempts, there remains a need in the art for a method of converting syngas to higher alcohols that offers further improved selectivity to, in particular, ethanol.